Germ cell apoptosis is the final common pathway of injury following exposure to toxicants that target the interacting cell types necessary for successful spermatogenesis. Model toxicants have been developed as functional probes of each of these interacting cell types: 2,5-hexanedione, carbendazim, and mono-(2-ethylhexyl)phthalate target Sertoli cells, ethane-1,2-dimethane sulfonate targets Leydig cells, and x-irradiation targets germ cells. Over the past quarter century, many labs have contributed to a rich database describing how each of these model toxicants acts one at a time to produce testicular injury. However, real world exposures, like those occurring at Superfund sites and Brownfields in Rhode Island, involve complex mixtures of hazardous chemicals. This project takes the next step toward mechanistic understanding of complex exposures by combining these model testicular toxicants in a novel co-exposure paradigm. 2,5-Hexanedione exposure is characterized by a 3-week prodromal phase followed by the rapid onset of Sertoli cell dysfunction and germ cell loss. In preliminary experiments, we show that early during 2,5-hexanedione exposure, the seminiferous epithelium is highly susceptible to co-exposure toxicity by carbendazim, a Sertoli cell toxicant with the same subcellular target as 2,5-hexanedione, even though each toxicant alone produces only a modest injury response. This 2,5-hexandeione-induced sensitization of the seminiferous epithelium is exploited by model toxicant co-exposure to test the following working hypothesis: the extent of co-exposure synergy following 2,5-hexanedione priming of the seminiferous epithelium depends upon targeting and shared molecular perturbations. We address this hypothesis by pursuing these Specific Aims: 1) Characterize the dose response and time dependence of 2,5-hexanedione-induced sensitization to carbendazim co-exposure 2) Determine the dependence of the co-exposure response to model toxicants in the 2,5-hexanedione-primed testis on cellular and subcellular targeting 3) Use gene chips to obtain molecular fingerprints of testicular sensitization and the model toxicant injury responses to predict co-exposure toxicity.